1. Field of the Invention
The present invention relates to the control of an air conditioning system for an automotive vehicle, and more specifically to an apparatus for controlling an air intake door, which in turn regulates the intake of air from outside the vehicle and of air recirculated within the vehicle. In particular the invention concerns an air intake door control system in which, under the control of a fully automatic temperature control device, the air intake door can be switched between positions in which outside air only is admitted, recirculated air only is admitted, or a mixture of outside and recirculated air is admitted; quite independently of the fully automatic temperature control device, however, it is arranged that if required, the air intake door can be forced into the position where only recirculated air is admitted.
2. Summary of the Prior Art
A typical conventional air conditioning system is provided inter alia with a temperature regulation means, a mode selector with settings such as "OFF", "AIR-CONDITIONER", "HEATER" and so forth, and a means for selecting the air intake from outside air, recirculated air or a mixture of the two. Automatic control systems are known, in which the selection between outside air, recirculated air or a mixture is carried out by an air intake door which is moved mechanically within the air intake passage by for example a vacuum actuator.
For example, the control of the air intake door has been carried out such that when a manually operated temperature control lever disposed within the vehicle passenger compartment is operated to select a desired temperature for the air to be outputted into the compartment a temperature signal is generated in an electric circuit. The circuit resistance includes, the combined resistance of an outside air temperature sensor, an inside air temperature sensor and a temperature setting potentiometer (set by the temperature control lever) connected in series. The temperature signal is fed to a differential amplifier whose output current causes a transducer to drive a vacuum power servomechanism to open or close an air mix door which controls the ratio of the amount of warm air to cold air outputted into the passenger compartment. According to the movement of the air mix door, the mode selector is automatically operated to select a desired control mode, such as "HEATER" to supply vacuum from a vacuum source through a manually operated selector which manually controls the supply of vacuum and atmospheric air to the air intake door vacuum actuator.
In the control system it is quite possible for there to be several other modes of operation in addition to those outlined above. These include the so-called "maximum cooling" mode in which only recirculated air, which is thus already substantially cooled, is used as intake to the air conditioner. Another such mode is a split level mode of operation in which cooled air is delivered to the windshield to defrost it, while warm air is delivered downward to the passenger compartment in order to warm the occupants' feet. In a special-purpose defrost mode, only outside air may be admitted, in order more quickly to clear the windshield. It will thus be readily apparent that the mechanism required, in terms of vacuum switching devices and the like, to operate the air intake door vacuum actuator is relatively complicated.
It is conventionally known to provide, either as part of the mode selector swich or separately, a switching means whereby an operator, which may be the driver, or possibly of course a passenger, may manually select outside air or recirculated air. In particular, there are occasions on which it is very desirable to be able to shut off rapidly the intake of outside air if the air outside the vehicle is suddenly undesirable. This will occur for example when the vehicle is passing a section of unsurfaced road which is dusty, or an agricultural installation, such as a pig farm, or factory, such as a chemical works, which gives off a generally unpleasant odor.
In a system with an automatic control as described earlier, therefore, it is no simple matter to provide a switch to cut off the intake of outside air, and commonly proposed conventional systems include for example electromagnetically operated valves in the pipes supplying vacuum to the air intake door vacuum actuator which are switched electrically to change the supply lines to the air intake door vacuum actuator to whichever state, vacuum or atmospheric pressure, and sets the air intake door in the position to admit only recirculated air. Since the air intake door normally has at least three positions, viz. outside air, recirculated air, and mixture, at least two supply lines are required to the air intake door vacuum actuator, and thus at least two electromagnetically operated valves are, of necessity, incorporated into the system. Such electromagnetically operated valves are expensive items, and this expense, together with the necessity for a controlling electrical circuit renders such a conventional system less than optimally desirable; it goes without saying that these are not the only drawbacks and that other factors such as the power consumed by electromagnetically operated valves will spring instantly to the minds of those acquainted with the art.
It is accordingly the object of the present invention to provide a new and improved system for controlling the air intake door which overcomes the problems of the above mentioned variety.